Children with specific impairment often show an especially serious limitation in the use of "grammatical morphemes." These difficulties include problems with bound morphemes such as the past tense and third person singular inflections, and function words such as articles and auxiliaries. The purpose of this project is to explore the possible bases of these grammatical morpheme limitations, and to examine how such limitations may hinder other aspects of these children's language development. To accomplish this general goal, specifically-language-impaired (SLI) children's comprehensive and use of grammatical morphemes will be examined across three different languages (English, Hebrew, Italian) within the framework of a comprehensive theory of language learnability. Experiments 1 and 2 center on SLI children's comprehension and use of morphemes that are low in "phonetic substance," that is, morphemes that are nonsyllabic consonant segments or unstressed syllables. The focus of Experiment 3 is whether low-phonetic-substance morphemes are especially difficult for SLI children because these children have problems discriminating linguistic material that is not perceptually salient. The final experiment explores the hypothesis that SLI children learn other aspects of language more slowly in part because they are unable to rely on low-phonetic- substance morphemes as parsing aids when presented with new linguistic material.